1. Field of the Invention
The present invention relates to fabrication of a semiconductor device, and more particularly to a method for fabricating a hetero-junction bipolar transistor in which a base parasitic capacitance is fully reduced by using a metallic silicide as a base.
2. Description of the Prior Art
As integration of a semiconductor device is improved higher and a semiconductor device is scaled down in size, the operation speed of such semiconductor device can be improved, but the operation characteristic thereof is restricted. It is because dopants injected into emitter and base of the device are increased. To enhance operation characteristics of a semiconductor device, several types of hetero-junction bipolar transistors have been developed. Typical one of them has a SiGe base which is substituted for a silicon base, and has a characteristic of narrowing in energy band gap and grading dependently upon Ge content of the SiGe base.
Recently, a hetero-junction bipolar transistor, which allows a metallic silicide base such as TiSi.sub.2 to be substituted for a polysilicon base, is actively researched so as to reduce a parasitic capacitance of a base region or between emitter and base regions, and therefore enhance performance thereof.
The fabricating method of the prior art hetero-junction bipolar transistor will be described below with reference to FIG. 1.
First, after forming a collector region 2 and a collector sinker 4 electrically isolated by an oxide layer 3 on a silicon substrate 1, a SiGe base layer 9 is formed on the collector region 2 by a selective epitaxy growth. Next, after deposition of an insulating layer 7 thereon, the insulating layer is patterned to define an emitter region. Then, an emitter 9 is formed on the base layer and a side wall 8 is formed at both edges of the emitter 9.
Subsequently, after coating a metal thin film only on an inactive region of the base layer 5, an annealing is performed to form a base electrode thin film 6 composed of a metallic silicide.
Finally, after exposing a portion of the base electrode thin film 6, an interconnection electrode 11 is formed on the exposed portion. As a result, fabrication of the prior art hetero-junction bipolar transistor is completed.
In the prior art bipolar transistor, since the base layer 5 is composed of an intrinsic SiGe film, an emitter injection efficiency is improved. Since the base electrode thin film 6 is composed of a metallic silicide film, a parasitic resistance of the thin film 6, i.e. a parasitic resistance of the metallic silicide itself or a contact resistance between the metallic silicide and the interconnection electrode, can be reduced.
However, since a metallic silicide constituting the base electrode thin film 6 is formed by annealing of the base layer 5 and a metal deposited thereon, there arises the problem that a loss of film thickness of the base layer 5 occurs. Thus, the metallic silicide 6, which is used to reduce a parasitic resistance of the base, becomes thinner. It is well-known in the art that the thinner the base layer becomes, the larger the parasitic resistance of the base layer occurs.
In addition, since a region reacting with a metal to form the metallic silicide is the ultra-thin base layer 5 of about 500 .ANG. in thickness, thickness of the metallic silicide 6 is further limited. For this limitation, surface resistance of the metallic silicide 6 is increased, and therefore reduction of parasitic resistance in the base is seriously restricted by thickens of the metallic silicide film 6.